Blind riveting is a most useful joining method which was originally developed at the end of the 19th century. The method is particularly applicable to the joining of metal sheets.
In conventional blind riveting, a hole is predrilled or punched through e.g. two overlapping metal sheets which are to be joined, and then the blind rivet is placed in the hole. The blind rivet comprises a shaft, and a sleeve or tubular rivet which is slipped over the shaft and has a tubular shank and a rivet head. Outside the end of the rivet shank, the shaft is formed with a thickened portion which, when tractive force is applied to the shaft, is pulled into and expands the adjacent end portion of the shank. As the application of the tractive force proceeds, the ultimate strength of the shaft is achieved and the shaft breaks inside the sleeve, where-upon the riveted joint is completed.
One drawback of conventional blind riveting is that predrilling is necessary, which requires two tools--one drilling machine and one drawing tool, a pair of so-called rivet pliers. One proposal of overcoming this draw-back is described in document DE-A-2,554,577 which discloses a self-drilling blind rivet. The self-drilling function is achieved by providing the lower end of the shaft with a drilling bit. In the driving-in operation, the shaft and the sleeve are simultaneously rotated by torque-absorbing means in the form of flat surfaces on the shaft.
However, by means of the blind rivet according to the DE-A-2,554,577, no pressure-tight joint is provided, since there is usually such a play between the sleeve and the drilled hole that air may pass. Moreover, in actual practice it often happens that the drilling bit comes loose and falls off from the deformed sleeve, whereupon air may flow freely through the sleeve. This causes further inconvenience when riveting, for example, parts of a ventilation duct, since the torn-off drilling bit will get into the duct.
U.S. Pat. No. 4,990,042 which discloses a self-drilling blind rivet certainly discusses fluid-tight riveted joints, but no special arrangements are suggested for preventing that the drilling bit comes loose during or after the riveting operation. The main object of the invention according to this US patent is to provide a special weakening of the material of the shaft such that a considerable amount of tractive force may be applied, and consequently the sleeve is completely deformed before the shaft breaks. This is achieved by means of a number of inclined flutes in the shaft.